The long-term objective of the current application is to elucidate novel molecular networks underlying mammalian meiosis. Meiosis, a process unique to germ cells, involves pairing, synapsis, recombination, and segregation of homologous chromosomes. Genetic abnormalities resulting from meiosis are a leading cause of birth defects and infertility in humans. The structural and functional properties of meiotic chromatin, which undergoes extensive reorganization, are undoubtedly the central theme of meiosis. Despite rapid progress in understanding meiosis, the complex interplay between chromatin organization and meiotic processes (such as synapsis and recombination) remains largely unknown. In particular, the progress in understanding mammalian meiosis has lagged far behind meiotic studies in other model organisms, due to several critical barriers: high cost, long duration, and the lack of sequence conservation of many meiosis-specific proteins across distant species. To overcome these roadblocks, we have developed an innovative proteomics approach to systematically identify a large number of uncharacterized mammalian meiotic chromatin-associated proteins in mice. The current application is to investigate the role of MEIOB, a novel meiosis-specific protein identified in our proteomics screen, in regulating meiotic recombination and chromosomal synapsis in mice. MEIOB is a sequence paralogue of the ubiquitously expressing RPA1. We find that MEIOB binds to single-stranded DNA (ssDNA) and exhibits 3' to 5' ssDNA-specific exonuclease activity. MEIOB forms distinct foci on meiotic chromosomes. In testes, MEIOB forms a complex with RPA and SPATA22. These proteins colocalize in foci on meiotic chromosomes. Furthermore, Meiob-null mutant mice of both sexes exhibit failures in meiotic recombination and chromosomal synapsis, resulting in sterility. Our results strongly support that MEIOB functions at a distinct step -second end capture - in meiotic recombination. Our specific aims are: 1) to examine promotion of single-stranded DNA annealing by MEIOB and its associated proteins; 2) to determine the requirement of MEIOB ssDNA-binding activity for meiotic recombination in vivo; 3) to elucidate the role of RPA1, a ubiquitously expressed ssDNA-binding protein, in meiotic recombination. Together, our studies will uncover the molecular mechanisms underlying the regulation of meiotic recombination in mammals by two related ssDNA-binding proteins MEIOB and RPA1, and will provide insights into the etiology of infertility and birth defects in humans.